Re: [PATCH dwarves v2 1/4] btf_encoder: Add .BTF section using libelf

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Giuliano Procida <gprocida@google.com>
Cc: dwarves@vger.kernel.org,
	"Arnaldo Carvalho de Melo" <acme@kernel.org>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Matthias Männich" <maennich@google.com>,
	kernel-team@android.com, "Kernel Team" <kernel-team@fb.com>,
	bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH dwarves v2 1/4] btf_encoder: Add .BTF section using libelf
Date: Wed, 3 Feb 2021 20:10:13 -0800	[thread overview]
Message-ID: <CAEf4BzYxfO72ozDtjjXynewfQv_ZLvVEFWrEHwro7J1uwMy-Kw@mail.gmail.com> (raw)
In-Reply-To: <20210201172530.1141087-2-gprocida@google.com>

On Mon, Feb 1, 2021 at 9:26 AM Giuliano Procida <gprocida@google.com> wrote:
>
> pahole -J uses libelf directly when updating a .BTF section. However,
> it uses llvm-objcopy to add .BTF sections. This commit switches to
> using libelf for both cases.
>
> This eliminates pahole's dependency on llvm-objcopy. One unfortunate
> side-effect is that vmlinux actually increases in size. It seems that
> llvm-objcopy modifies the .strtab section, discarding many strings. I
> speculate that is it discarding strings not referenced from .symtab
> and updating the references therein.
>
> In this initial version layout is left completely up to libelf and
> indeed offsets of existing sections are likely to change.
>
> Signed-off-by: Giuliano Procida <gprocida@google.com>
> ---
>  libbtf.c | 134 ++++++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 88 insertions(+), 46 deletions(-)
>
> diff --git a/libbtf.c b/libbtf.c
> index 81b1b36..5b91d3a 100644
> --- a/libbtf.c
> +++ b/libbtf.c
> @@ -698,6 +698,7 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>         uint32_t raw_btf_size;
>         int fd, err = -1;
>         size_t strndx;
> +       void *str_table = NULL;
>
>         fd = open(filename, O_RDWR);
>         if (fd < 0) {
> @@ -740,74 +741,115 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>         }
>
>         /*
> -        * First we look if there was already a .BTF section to overwrite.
> +        * First we check if there is already a .BTF section present.
>          */
> -
>         elf_getshdrstrndx(elf, &strndx);
> +       Elf_Scn *btf_scn = 0;

NULL, not 0

>         while ((scn = elf_nextscn(elf, scn)) != NULL) {
>                 shdr = gelf_getshdr(scn, &shdr_mem);
>                 if (shdr == NULL)
>                         continue;
>                 char *secname = elf_strptr(elf, strndx, shdr->sh_name);
>                 if (strcmp(secname, ".BTF") == 0) {
> -                       btf_data = elf_getdata(scn, btf_data);
> +                       btf_scn = scn;
>                         break;
>                 }
>         }
>
> -       raw_btf_data = btf__get_raw_data(btf, &raw_btf_size);
> -
> -       if (btf_data) {
> -               /* Exisiting .BTF section found */
> -               btf_data->d_buf = (void *)raw_btf_data;
> -               btf_data->d_size = raw_btf_size;
> -               elf_flagdata(btf_data, ELF_C_SET, ELF_F_DIRTY);
> +       Elf_Scn *str_scn = elf_getscn(elf, strndx);
> +       if (!str_scn) {
> +               fprintf(stderr, "%s: elf_getscn(strndx) failed\n", __func__);

no elf_errmsg(elf_errno()) here? BTW, this form is very common (and a
bit verbose), so how about having a local macro that would make this
shorter, e.g.:

elf_error("elf_getscn(strndx) failed"); ?

> +               goto out;
> +       }
>
> -               if (elf_update(elf, ELF_C_NULL) >= 0 &&
> -                   elf_update(elf, ELF_C_WRITE) >= 0)
> -                       err = 0;
> -               else
> -                       fprintf(stderr, "%s: elf_update failed: %s.\n",
> -                               __func__, elf_errmsg(elf_errno()));
> +       size_t dot_btf_offset = 0;
> +       if (btf_scn) {
> +               /* Existing .BTF section found */
> +               btf_data = elf_getdata(btf_scn, NULL);
> +               if (!btf_data) {
> +                       fprintf(stderr, "%s: elf_getdata failed: %s\n", __func__,
> +                               elf_errmsg(elf_errno()));
> +                       goto out;
> +               }
>         } else {
> -               const char *llvm_objcopy;
> -               char tmp_fn[PATH_MAX];
> -               char cmd[PATH_MAX * 2];
> -
> -               llvm_objcopy = getenv("LLVM_OBJCOPY");
> -               if (!llvm_objcopy)
> -                       llvm_objcopy = "llvm-objcopy";
> -
> -               /* Use objcopy to add a .BTF section */
> -               snprintf(tmp_fn, sizeof(tmp_fn), "%s.btf", filename);
> -               close(fd);
> -               fd = creat(tmp_fn, S_IRUSR | S_IWUSR);
> -               if (fd == -1) {
> -                       fprintf(stderr, "%s: open(%s) failed!\n", __func__,
> -                               tmp_fn);
> +               /* Add ".BTF" to the section name string table */
> +               Elf_Data *str_data = elf_getdata(str_scn, NULL);
> +               if (!str_data) {
> +                       fprintf(stderr, "%s: elf_getdata(str_scn) failed: %s\n",
> +                               __func__, elf_errmsg(elf_errno()));
>                         goto out;
>                 }
> -
> -               if (write(fd, raw_btf_data, raw_btf_size) != raw_btf_size) {
> -                       fprintf(stderr, "%s: write of %d bytes to '%s' failed: %d!\n",
> -                               __func__, raw_btf_size, tmp_fn, errno);
> -                       goto unlink;
> +               dot_btf_offset = str_data->d_size;
> +               size_t new_str_size = dot_btf_offset + 5;

5 is a bit magical, maybe use sizeof(".BTF") or a dedicated constant?

> +               str_table = malloc(new_str_size);
> +               if (!str_table) {
> +                       fprintf(stderr, "%s: malloc (strtab) failed\n", __func__);
> +                       goto out;
>                 }
> -
> -               snprintf(cmd, sizeof(cmd), "%s --add-section .BTF=%s %s",
> -                        llvm_objcopy, tmp_fn, filename);
> -               if (system(cmd)) {
> -                       fprintf(stderr, "%s: failed to add .BTF section to '%s': %d!\n",
> -                               __func__, filename, errno);
> -                       goto unlink;
> +               memcpy(str_table, str_data->d_buf, dot_btf_offset);
> +               memcpy(str_table + dot_btf_offset, ".BTF", 5);

same about magical 5

> +               str_data->d_buf = str_table;
> +               str_data->d_size = new_str_size;
> +               elf_flagdata(str_data, ELF_C_SET, ELF_F_DIRTY);
> +
> +               /* Create a new section */
> +               btf_scn = elf_newscn(elf);
> +               if (!btf_scn) {
> +                       fprintf(stderr, "%s: elf_newscn failed: %s\n",
> +                       __func__, elf_errmsg(elf_errno()));
> +                       goto out;
> +               }
> +               btf_data = elf_newdata(btf_scn);
> +               if (!btf_data) {
> +                       fprintf(stderr, "%s: elf_newdata failed: %s\n",
> +                       __func__, elf_errmsg(elf_errno()));
> +                       goto out;
>                 }
> +       }
>
> -               err = 0;
> -       unlink:
> -               unlink(tmp_fn);
> +       /* (Re)populate the BTF section data */
> +       raw_btf_data = btf__get_raw_data(btf, &raw_btf_size);
> +       btf_data->d_buf = (void *)raw_btf_data;
> +       btf_data->d_size = raw_btf_size;
> +       btf_data->d_type = ELF_T_BYTE;
> +       btf_data->d_version = EV_CURRENT;
> +       elf_flagdata(btf_data, ELF_C_SET, ELF_F_DIRTY);
> +
> +       /* Update .BTF section in the SHT */
> +       GElf_Shdr btf_shdr_mem;
> +       GElf_Shdr *btf_shdr = gelf_getshdr(btf_scn, &btf_shdr_mem);
> +       if (!btf_shdr) {

btf_shdr just points to btf_shdr_mem, no? This duplication is not
pretty, why not:

GElf_Shdr btf_shdr;
if (!gelf_getshdr(btf_scn, &btf_shdr_mem)) { ... }

And then use btf_shdr. everywhere below

> +               fprintf(stderr, "%s: elf_getshdr(btf_scn) failed: %s\n",
> +                       __func__, elf_errmsg(elf_errno()));
> +               goto out;
> +       }
> +       btf_shdr->sh_entsize = 0;
> +       btf_shdr->sh_flags = SHF_ALLOC;

this is wrong, making .BTF allocatable should be an opt-in, not all
applications need to have a loadable .BTF section. Plus this patch
doesn't really make it loadable, so SHF_ALLOC should be updated in the
later patch. And I don't think we'll use that for vmlinux BTF or
kernel module BTFs either, because there is still going to be linker
script involved.

> +       if (dot_btf_offset)
> +               btf_shdr->sh_name = dot_btf_offset;
> +       btf_shdr->sh_type = SHT_PROGBITS;
> +       if (!gelf_update_shdr(btf_scn, btf_shdr)) {
> +               fprintf(stderr, "%s: gelf_update_shdr failed: %s\n",
> +                       __func__, elf_errmsg(elf_errno()));
> +               goto out;
> +       }
> +
> +       if (elf_update(elf, ELF_C_NULL) < 0) {
> +               fprintf(stderr, "%s: elf_update (layout) failed: %s\n",
> +                       __func__, elf_errmsg(elf_errno()));
> +               goto out;
> +       }
> +
> +       if (elf_update(elf, ELF_C_WRITE) < 0) {
> +               fprintf(stderr, "%s: elf_update (write) failed: %s\n",
> +                       __func__, elf_errmsg(elf_errno()));
> +               goto out;
>         }
> +       err = 0;
>
>  out:
> +       if (str_table)
> +               free(str_table);
>         if (fd != -1)
>                 close(fd);
>         if (elf)
> --
> 2.30.0.365.g02bc693789-goog
>